A differential pair is one of the most commonly used circuits in analog design. A differential pair comprises 2 transistors sharing a common source and may be employed as the input stage to a current amplifier, such as an operational transconductance amplifier (OTA). An OTA is a voltage controlled current source whose differential input voltage produces and output current. A differential pair is popular because it achieves much lower offsets and much higher rejection of common mode disturbances over a conventional, single-ended input stage. Where input common mode ranges allow, p-channel metal oxide semiconductor (PMOS) differential pairs are preferred over their N-channel metal oxide semiconductor (NMOS) counterparts. One reason that a PMOS differential pair is preferred is because the source and bulks of a PMOS transistor can be shorted in a conventional device to eliminate supply disturbances coupling through the PMOS bulks to the OTA output. This makes a PMOS differential pair more robust to supply disturbances than its NMOS counterpart.
However, it is not possible to short the source and bulks of a PMOS transistor when using a modern “triple well” or “deep N-well” (DNW) process. Modern triple well or DNW processes provide deep N-well isolation which enable sensitive analog blocks (such as an OTA) to be isolated from noisy substrates. This is advantageous for system on chip (SOC) ICs where sensitive analog blocks co-exist on the same piece of silicon as noisy high frequency digital blocks. However, the problem with deep N-well isolation is that the DNW is connected to power. Because the DNW is shorted to the N-well (NW) of a PMOS, the bulk of a PMOS is therefore also connected to power and hence cannot be shorted to its source. As a result, a PMOS differential pair with DNW isolation experiences the same inferior rejection of supply disturbances as an NMOS differential pair.
Supply disturbances at the bulks of an OTA's input differential pair couple currents across the bulk/gate capacitances (Cbg) to the OTA differential pair gate inputs. From here, these currents are converted into voltages by flowing across impedances. If these impedances are not equal, a differential voltage will develop at the OTA inputs to which its output will react. The degree to which the OTA output moves in relation to the supply disturbances is quantified by its power supply rejection ratio (PSRR). The PSRR is the inverse ratio of the disturbance on the supply line to that of the output, and is typically given in units of decibels (dB).
Accordingly, circuits and methods that improve the performance of a differential pair are beneficial.